Using a directory of files rather than a flatfile

[stuartpb]

• They're easier to manipulate and clobber
• They're less prone to getting noise/errors from something not doing printf '%q'
• It lets us provide the environment as a volume to the buildpack the way Heroku does it (Providing the environment to buildpacks plushu-buildstep#3).

[stuartpb]

Also, this kind of suggests environment variables are added by another means than a script in .profile.d; although it would be a break from Dokku, we could then change plushu-release-env to provide config vars via --env on the app run command. I'm kind of ambivalent about that.

[stuartpb]

Of course, the current way can be done via --env-file - --env would actually be more complicated.